The Effect of Total Binding Capacity of Thyroxine Binding Globulin on the Free Thyroxine Index

摘要

In search of a definite source of misleading free thyroxine index (FT₄I), the relationships between in vitro thyroid testing results and thyroxine-binding globulin (TBG) capacities were reexamined in sera from a population with a relatively high prevalence of serum TBG alterations. Sera from 21 subjects with different total thyroxine-binding globulin capacities (TTBG), were loaded with graded amounts of thyroxine (T₄) and assayed for T₄, T₃ uptake (T₃U), TTBG, and free T₄ concentration (FT₄). Serum T₄, T₃U, and the calculated FT₄ index (FT₄I) were able to separate efficiently the samples according to their FT₄, but their respective normal ranges varied with TTBG. Interpretation of the results of the in vitro tests, in the light of TTBG, greatly improved their operating characteristics in the study of 141 patients with a high prevalence of TBG alterations. The misleading FT₄I is not the outcome of reduced intrinsic sensitivities of the in vitro tests, but a consequence of a shift of their normal ranges caused by a change of TTBG. By estimating TTBG from the values of T₄ and T₃U, this problem is easily solved without adding cost.